These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Broken Base: It's not volatile, unlike 99% of the internet, but Moomin fans can't seem to decide which foreign dub of the series is best. English? Finnish? Polish? French?
Also, whether the heavily Bowdlerized anime series is an acceptable adaptation at all. Especially older fans (i.e. the parents of the generation who saw the series in the 90's) seems to be more hostile towards it.
The anime series was popular in Hawaii, airing on the local UPN affiliate.
It's also popular in Poland (got a rerun in 2006) and in Israel (basically everyone who was born between the mid-eighties to the early nineties grew up watching it), but it was no doubt most popular in Finland, where it reruns regularly. Heck, you KNOW a series is popular in another country when they manage to dub EVERY SINGLE EPISODE and the Big Damn Movie on top of it!
Memetic Mutation: EVERYONE in Scandinavia and Poland are afraid of the Groke. No exceptions. The show was IMMENSELY popular, and everyone of the generation that watched the 90's animated series remembers that freaking thing. It was one of the ONLY times in Scandinavia that a show was pulled off the air (for a time) because parents called and complained about it being too scary. That almost never happens there.
Nightmare Fuel: The Groke was this to any child who ever watched the show. Especially in Finland, Poland, France and Japan, where she has the most terrifying voices. Also, making her mute in the English dub is creepy, too. It was greatly helped by the build-up and suspense in her introduction episode, along with the horrifying music that played throughout her scenes.
The Lady of the Cold, with her creepy singing and ability to freeze people where they stand.
She's worse in the book. Anyone frozen by her dies instantly, they can't just be thawed up again like in the anime.
Don't forget that purple abomination Moomin turns into when he's transformed by that purple hat in the first episode! God, the fact that Moomin's voice is in tact as he transforms makes it all the more unsettling to look at AND listen to!
The Hattifatteners are also creepy, they always travel in large groups and doesn't seem dangerous at first; however they are terryfying when they get angry and when the electrify themselves, which makes them dangerous to get close.
Not to mention Thingumy and Bob. You might presume they're female like their real life counterparts but the English names are rather confusing, especially "Bob". And the original names, Tofslan & Vifslan, give even less of a hint of their genders. The original Swedish novel never mentions their genders. But it does it so skilfully that you won't notice until you start wondering what their genders are.
For the readers of the Finnish translation this goes Up to Eleven. Unlike Swedish (and English) Finnish doesn't distinguish gender grammatically, not even in pronouns. Thus there's no he/she distinction. One can read all the books and all the comics and watch all the adaptations in Finnish and never find out the genders of the more ambiguous characters. Hence the confusion of many Finns when they find out the Groke was female all along (not helped by the voice given to her in the Finnish dub of the animated series). Thingumy and Bob are similarly difficult to figure out. Too-Ticky is slightly easier since her name sounds a lot like her real life counterpart's Tuulikki, which is a Finnish woman's name. (And even then people can be confused. While the -kki ending is more common in feminine names, it's by no means a strict rule.)
What Do You Mean, It's for Kids? / What Do You Mean, It's Not for Kids?: Uniquely, the Moomins fit both tropes. The later books in the series aren't really children's books (something which Tove Jansson herself acknowledged), touching on a lot of themes that kids won't understand. The newspaper comic, while less dark, contains a lot of satire directed primarily at adults: The Moomins' bohemian lifestyle is often mocked, with the characters played for fools in a lot of subtle ways, but other, more "normal" lifestyles are frequently held up for examination and found to be even more foolish. At the same time, earlier books and works which are primarily directed at children, contain some pretty dark undertones and subtle adult humor. The '90s animated series and its spin-offs are more clearly aimed at children (though even there the undertones aren't completely gone AND The Groke is still there), which is the main reason why so many older Moomin fans don't like it.
The Woobie: The Groke especially gets this treatment in the last books.
Sniff, especially in the books. Completely averted in the comic strip, where he's more a Karma Houdini — and so, in the animated series, which takes cues from both books and comic strip, he's a Jerkass Woobie.
Sorry-Oo was basically designed to be this.
Ninny, the invisible girl who was abused by her aunt
Fillyjonk's children. They are not allowed to have fun!